Laser absorption spectroscopy has emerged as one of the most important applications for semiconductor quantum cascade (QC) lasers, particularly in the mid-infrared spectral regime where many gases of interest exhibit strong absorption features. Single-mode laser sources can selectively target absorption lines of key atmospheric gases and their less abundant isotopologues in order to study the structure and evolution of Earth and even other solar system bodies. For environmental safety and monitoring, laser-based sensors have the potential to match or exceed the sensitivity of electrochemical sensors presently used in many applications, and laser-based detectors are capable of long-duration maintenance-free operation without consumable components.
To realize an effective laser-based absorption spectroscopy instrument, the laser source should produce stable single-mode emission with enough tunability to span at least one target gas absorption line. This has prompted the development of several schemes for suppressing multimode operation, the most compact and mechanically robust of which employs an integrated distributed-feedback (DFB) grating. Over the past several years, DFB QC lasers have been demonstrated using both loss-coupled metallic gratings and index-coupled semiconductor structures. For low power operation, which is particularly important for portable gas detection instruments, buried heterostructure devices with etched index-coupled gratings have been the most successful; however, fabrication of buried gratings relies on epitaxial regrowth, which adds significant processing complexity and capital equipment costs.